Allelophathic Effect Of Sargassum Thunbergii On Skeletonema Costatum And Its Response To CO₂ Enrichment

Harmful algae blooms (HABs) has been considered as a global ecological disaster which severely threaten the sustainable development of marine economy. Therefore, effective and environment-friendly methods of HABs mitigation are necessary. Recently, many researchers focus on the allelopathic effect of aquatic macroalgae on HABs causative algae which is deemed to be a promising method to control HABs. The steady enrichment of CO2 at unprecedented speed has become one of the seriously global environment problems. Thereby, it is important to investigate the effect of CO2 enrichment on marine algae and on the interaction between them, and the result will be useful to reveal the influence of global climatic change on marine ecosystem. In the present study, marine macroalga Sargassum thunbergii and HABs causative microalga Skeletonema costatum were selected to investigate the allelopathic effect of S. thunbergii on S. costatum as well as the response of the allelopathic effect to CO2 enrichment. The mechanisms of allelopathic effect between marine algae and its response to CO2 enrichment were also discussed, and the responsibility of using macroalgae to control HABs was evaluated. The results showed that:1. The allelopathic effect of Sargassum thunbergii on Skeletonema costatumThe fresh tissue and dry power of S. thunbergii showed significantly inhibitory effect on the population growth of S. costatum. The carrying capability (K) and growth rate (r) of S. costatum population decreased significantly (P<0.05). Furthermore, the inhibitory effect was strengthened with the increasing concentration of fresh tissue and dry power, and dry power at a higher concentration of 2.4 g FD·L-1 had lethal effect on S. costatum. The EC50 of fresh tissue of S. thunbergii was estimated as 3.5 g FW·L-1 by straight line interpolation and the EC50 of dry power was 1.8 FD·L-1. The results confirmed that the macroalga S. thunbergii had allelopathic effect on S. thunbergii. The aqueous extract of S. thunbergii had significantly inhibitory effect on the population growth of S. costatum, and the inhibitory effect strengthen with the increasing concentration of aqueous extract. And the culture medium filtrate of S. thunbergii also had significantly inhibitory and lethal effect on S. costatum. The results indicated that S. thunbergii secreted allelopathic substance to influence S. costatum.The boiled aqueous extract and culture medium filtrate of S. thunbergii also had significant inhibitory effect on S. costatum. Culture medium filtrate of S. thunbergii appeared remarkably inhibitory effect on S. costatum under either initial filtrate addition or semi-continuous addition, and all S. costatum were killed at the end of the experiment. The results indicated that the allelopathic substances secreted by S. thunbergii were relative stable.2. Response of allelopathic effect of S. thunbergii on S. costatum to CO2 enrichmentThe fresh tissue, dry power and aqueous extract of S. thunbergii which were culture under CO2 enrichment condition obviously led to a decrease in population density and the value of K and r(P<0.05) compared with them cultured under normal condition. The results demonstrated that CO2 enrichment resulted in an increase in allelopathic effect of S. thunbergii on S. costatum.The fresh tissue, dry power and aqueous extract of S. thunbergii cultured under CO2 enrichment condition had a stronger allelopathic effect compared with them cultured under normal condition, but the allelopathic effect of culture medium filtrate decreased remarkably. Accordingly, it was speculated that the action way of allelopathic effect might be changed by CO2 enrichment, which changed from secreting allelopathic substance to direct cell contact transmission.